(i) Field of the Invention
This invention relates to plasma discharge devices, particularly but not exclusively adapted for use in mass spectrometers using electrostatic mass filters.
(ii) Prior Art
Mass spectrometers utilizing electrostatic mass filters such as quadropole mass filters are well known. Such spectrometers are arranged so that ions of a gas or vapour to be analysed are generated in an ion source and directed into the mass filter, filtered ions being detected by a suitable detector.
The ion source used in a mass spectrometer of the above kind may take in a number of forms, but in the past it has been customary to use electron impact devices. However, plasma discharge devices, such as the known "Penning discharge" device, are known to have desirable characteristics making them particularly suitable for use as ion sources, but these have not found favour because of an unsatisfactory energy distribution of ions produced from such devices. In particular, substantial quantities of high energy ions are normally produced and these tend to travel through the mass filter regardless of mass, so resulting in a downgrading of the filtering characteristics of the mass filter. Solutions to this problem have been proposed. For example, an effective mass spectrometer using a particular form of plasma discharge device is described in the publication NASA CR-1475 of the National Aeronautical Aeronautics and Space Administration, entitled "A Cold Cathode Ion Source Mass Spectrometer Employing Ion Counting Techniques" by F. L. Torney Jr., P. Blum, P. Fowler and J. R. Roehrig. However, this device is relatively complex.
Against the above background, an object of the present invention is to provide an improved ion source which is arranged to limit the energy spread of ions produced thereby but which is of relatively simple construction.